1. Field of the Invention
The present invention relates to a fine particle removing apparatus which removes fine particles, especially flammable fine particles in exhaust gas from a diesel engine, a boiler, an incinerator or the like, and to a filter unit used in this fine particle removing apparatus.
2. Description of the Related Art
Various types of diesel particulate filters (DPFs) which collect harmful fine particles emitted from a diesel engine have been developed.
For example, Jpn. Pat. Appln. KOKAI Publication No. 8-826522 discloses a DPF comprising: a pipe formed of a non-magnetic material; a metallic filter which is arranged in this pipe formed of a non-magnetic material and in which many elongated exhaust gas paths are formed by regularly arranging many metallic members such as metal sheets or small-diameter metallic pipes; and a coil which is arranged on the outer periphery of this pipe formed of a non-magnetic material and to which a high-frequency current is supplied.
In this apparatus, an eddy current is induced in the surfaces of many metallic members which partition the elongated exhaust gas paths of the metallic filter by supplying a high-frequency current to the coil, and the metallic members are heated to a high temperature which is approximately 600° C. or above by Joule heat produced from this eddy current. When exhaust gas flows through these elongated exhaust gas paths, flammable fine particles in the exhaust gas come into contact with the high-temperature metallic members which partition the elongated exhaust gas paths, and hence the fine particles are burned.
However, this DPF constantly supplies a high-frequency current to the coil during the operation, and a large quantity of current is thereby consumed. Further, when the exhaust gas paths are elongated in order to efficiently burn flammable fine particles in the exhaust gas, the size of the entire apparatus is increased, the energy required for heating is increased, and hence combustion cannot be efficiently performed.
Furthermore, “ECO INDUSTRY” (CMC Publishing Co., Ltd., February 2001, p. 12–18) discloses a DPF manufactured by holding a sheet of ceramic fiber felt by a wire mesh heater from both sides to be formed into a plate-like shape, combining many the plate-like felt sheets and heaters to form a pleated filter element and accommodating this filter element in a casing. Two DPFs are arranged in parallel, exhaust flow paths are switched by using a control valve provided on the upstream side so that fine particles are collected on one hand and regeneration is carried out on the other hand, thereby always collecting fine particles. Regeneration of this DPF is performed by energizing the wire mesh heaters in each filter element and burning fine particles collected in the felt.
The DPF having the pleated filter elements is very beneficial in that the breakage of the filter elements due to the heat stress in regeneration is avoided and collection and regeneration of fine particles are possible irrespective of fuel properties, but the wire mesh heater formed of a thin metal is arranged on the surface of the ceramic fiber felt, and hence this wire mesh heater is always exposed to the exhaust gas and heated to a very high temperature at the time of regeneration. Therefore, the wire forming the wire mesh heater may be possibly disconnected. Furthermore, since the two DPFs are alternately used for collection and regeneration, the configuration and the combustion control become very complicated.
Thus, development of a DPF which has a compact configuration but can efficiently remove flammable fine particles in exhaust gas has been desired.